Zero Offset Head
When a blade wants to flap up or down, the forces involved are significant. This leads
to large moments being transmitted to the shaft, and these are prevented from
working by the use of flapping hinges, or some other form of flexible material.
Nevertheless, the blade flapping hinges do still transfer some moment to the shaft,
due to their offset there.
When flapping hinges are used, the offset, as depicted in the figure below in green,
determines
the amount of the moment that is transmitted to the shaft because of
disc tilting. When
the offset
is zero (second figure), the moment, 'm', cannot be
transmitted because the arm is zero. This design makes
it possible to use a less substantial shaft and rotorhead construction. This saves weight
and will, therefore,
require less engine power.
Another solution is to use a teetering hinge in a 2 bladed rotorhead. As the teetering
hinge pins coincide with the shaft axis, we again have a zero offset configuration.
Negative G
A disadvantage of zero offset heads is that they are prone to a phenomenon known
as negative g, from which the consequences can be quite catastrophic. Negative g is
present when the rotorthrust is upwards and 'pushing' the helicopter down. The helicopter
now accelerates downwards, and the pilot will experience less
body weight, hence the name, negative g.
Because of the design of a zero offset head, the helicopter can be pushed to any
site. Pushing it down is not a mechanically stable movement, unlike when the helicopter
is pulled upwards, against gravity. So, in these circumstances, there is a real danger
that the helicopter will change attitude dramatically in any direction. The
blades will then hit
the blade droop stops with enough force to severely damage the shaft. The
consequence will probably be break down. It is also quite possible that
the blades will cut through the tailboom.
Negative g with zero offset heads must be prevented at all costs. This is the pilot’s
responsibility.
Next topic > Blade Droop Stops
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Cyclic & Collective
- The title of this book leads me to wonder what more it will teach me in addition to its content about these two, most frequently used, helicopter input controls. As it turns out, the answer is: a lot more. Of course, all of the obligatory subjects like basic physics, rotor aerodynamics and helicopter performance are dealt with as well, as are piston engine and basic helicopter manoeuvres. Yet the scope of this book is actually much wider than one might initially think.
Firstly, it is divided into a 'beginners’ and an 'advanced’ section. This means that the book can treat more complex concepts in depth, even though the focus in the first section is directed more towards newcomers to the field.
Secondly, subjects like turbine engines, multi-engine helicopters and autopilots are also examined. This is particularly helpful, since these topics are not usually covered in the majority of helicopter books aimed at this target audience.
Thirdly, the book deals with many things that you will not normally find in a text book: helicopter related experiences and a great deal of interesting detail. This is the sort of information that can only be provided if you have flown a lot of different helicopters and have been working in this industry for some time. What’s more, this tone is amplified by the consistently narrative style of the book.
- 536 pages
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Art of the Helicopter (Hardback)
- Well structured text that covers many technical aspects. It starts with an introduction to helicopters, followed by a treatment of
the technical background needed when studying them. Thereafter, dynamics, rotor systems, engines and transmission are explained in detail.
The book concludes with a section on performance and other types of rotorcraft. Its main asset is that the text is technically and
theoretically very accurate, and rather than mathematics, its focus is always on enabling the reader to achieve an understanding of
helicopters from a technical or engineering point of view. The more technically orientated reader will love this work.
- 416 pages
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Principles of Helicopter Flight (Paperback)
- If you are wondering how a helicopter flies, and really want to comprehend the process, you have no choice but to delve into
aerodynamics. This means not only understanding which forces play a role and the laws of physics, but also being able to put it all
together and apply your knowledge to a rotating system consisting of a number of rotor blades. This is a demanding task and requires
some very hard work. It is, undoubtedly, worth the effort though, and will help you to become a better pilot. There are many books out
there to help you with this task of exploring the principles of helicopter flight, but they tend to fall into two camps:
populist and taking a rather simplistic approach, or highly technical and assuming the reader has a degree in mathematics.
This book is different, because it clearly explains the principles of flight in a step by step way that is easy for most
readers to follow. Further benefits are that a lot of attention is paid to flight manoeuvres and operations, and every chapter
concludes with questions as a study aid.
- 320 pages
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